The field of the invention relates to calibration and characterization of radiation sources in the near and far field and, more particularly, to cross-correlation of two ultrashort radiation pulses of different wavelengths.
Ultra-short laser pulses are used for many applications today. Characterizing a temporal profile of an intensity of the ultra-short laser pulse is often necessary. The process of calibrating often involves performing cross-correlation of an uncalibrated laser pulse with a laser pulse of shorter duration, hereinafter, a xe2x80x9cprobexe2x80x9d pulse. By cross-correlating, the temporal profile of the uncharacterized pulse can be determined as well as the temporal overlap between the uncharacterized and probe pulses. In performing cross-correlation, the probe pulse has a different wavelength than the uncalibrated laser pulse. Nonlinear optical processes have been developed to accomplish cross-correlations using harmonic generation to produce sum or difference frequency generation for two different wavelengths (cross-correlation).
Conventional cross-correlators involve focusing the uncalibrated and the probe pulse on a non-linear crystal. The two pulse paths are generally 5-10% off a normal plane defined by an input surface of the non-linear crystal such that each pulse is offset on a different side of the normal plane. The pulse paths are focused on non-overlapping points of the non-linear crystal""s input surface so that the paths of the two pulses converge at a spot within the non-linear crystal. Filters are placed on an output side of the non-linear crystal to block the strong primary beams of each radiation source. When properly configured, the non-linear crystal generates a sum frequency of the two incident laser pulses in a two-photon process. A photomultiplier or the like is placed at a point between the two primary beams leaving the non-linear crystal so that the intensity of the sum frequency may be measured. To achieve a phase matching angle so that the two radiation sources generate a sum frequency beam, the non-linear crystal must be tilted. Once the matching angle is achieved, the photomultiplier measures the sum frequency beam which resulted from the two-photon process.
As those skilled in the art can appreciate, the sum frequency beam requires a photon from the uncalibrated radiation pulse and a photon from the probe pulse to generate a photon in the sum frequency beam. So the sum frequency beam is only present when both radiation beams are properly focused on the surface of the non-linear crystal at the same time (i.e., both radiation beams temporally overlap). Since the characteristics of the probe pulse are well known, the portion of the sum frequency beam attributable to the uncalibrated pulse may be determined by analyzing a signal from the photomultiplier. The width of the probe pulse can be much smaller than the uncharacterized pulse so that thin slices of the uncharacterized pulse may be sequentially analyzed to determine the cross-correlation.
Conventional cross-correlators provide for adjusting the time at which the probe pulse converges with the uncharacterized pulse. A variable delay line or the like can be used so that the probe pulse can successively scan the uncharacterized pulse. In this way, slices of the uncharacterized pulse can be measured so that the intensity profile over time may be characterized.
As those skilled in the art can appreciate, the conventional technique of using a non-linear crystal to perform cross-correlation has many disadvantages. This technique requires a special non-linear crystal with a high non-linear coefficient for the incident light beams and sufficient transmission of the crystal material for the generated new frequencies to pass therethrough. Additionally, the crystal has to be phase matched to the incoming laser wavelengths, the laser beams have to be imaged accurately into the crystal material to obtain the high field strengths necessary for the non-linear process, and a sensitive detector (e.g. a photomultiplier) is required for measuring the weak intensity of the newly generated frequencies resulting from the non-linear process. Furthermore, the non-linear crystals are expensive and are not easily adaptable to uncharacterized pulses of many different wavelengths. Accordingly, there is a need for a cheaper and simpler way to perform cross-correlation of two radiation sources having different wavelengths.
In accordance with the present invention, a multi-wavelength cross-correlator for two radiation pulses is disclosed. Cross-correlation is performed in order to characterize a temporal profile of radiation pulses and a temporal overlap between two radiation pulses. Cross-correlations are used for calibrating a two radiation pulse system which assures both radiation pulses spatially and temporally overlap at a desired point. In one embodiment, a method for operating the cross-correlator is disclosed. The method includes the following steps:
(a) focusing the first radiation pulse on the photodiode (the first radiation pulse having a first intensity and a first wavelength);
(b) focusing the second radiation pulse on the photodiode where the first and second radiation pulses illuminate a common point on the photodiode (the second radiation pulse having a second intensity and a second wavelength), wherein the first and second wavelengths are different;
(c) converting the first and second radiation pulses into a photocurrent using the photodiode (where the product of the first and second intensities is proportional to the photocurrent); and
(d) detecting an amplitude of the photocurrent while varying the delay of at least one of the first and second radiation pulses.
Based upon the measured amplitude of the photocurrent, the product of the first intensity and the second intensity can be determined.
In another embodiment, a multi-wavelength cross-correlation system includes:
(a) a first radiation pulse;
(b) a second radiation pulse (the first and second radiation pulses having different wavelengths);
(c) a radiation detection means (e.g., a semiconductor), which converts energy from the first and second radiation pulses into a current, the first radiation pulse contacting a first area of the radiation detection means and the second radiation pulse contacting a second area of the radiation detection means with the first and second areas overlapping; and
(d) a current sensing means (e.g., a sensitive current meter and/or a chopper and lock-in amplifier) for determining an amplitude of the current.
The invention is useful for measurement of both femtosecond and picosecond laser pulses. In general, any commercial photodiode can be a used for this device, depending on the band gap energy and the wavelengths of the applied lasers in terms of their photon energy and intensity. Therefore the wavelength ranges that can be cross-correlated is greatly extended. Because a photodiode is much less expensive and easier to align than a non-linear crystal and a separate detector (e.g., a photomultiplier) and since the operation of the invention is much more straightforward, it can replace conventional non-linear optical processes for cross-correlation measurements. The cross-correlator is particularly effective when the first and second radiation pulses each have a wavelength ranging from the ultraviolet to the mid-infrared and a duration in the range from several picoseconds to a few femtoseconds.
Based upon the foregoing summary, a number of important advantages of the present invention are readily discerned. By using an inexpensive photodiode, the costs associated with the non-linear crystal and photomultiplier are avoided. Further, the two radiation source only need to spatially overlap on the photodiode, so the extensive alignment procedures of the conventional cross-correlators are unnecessary. Further still, the use of a photodiode increases the wavelengths which may be cross-correlated. Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.